Method and apparatus for tracking radially-dependent repeatable run-out

ABSTRACT

An apparatus and method for tracking radially-dependent repeatable run-out in a disc drive having a servo loop for positioning a head over a rotating disc is provided. The disc includes multiple tracks. Radially-dependent repeatable run-out control components for at least a subset of the multiple tracks are first determined. Data representative of the radially-dependent repeatable run-out control components for the subset of the multiple tracks is then stored. The stored data representative of the radially-dependent repeatable run-out control components is retrieved before settling on the target track, and subsequently used to follow the selected track.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. ProvisionalApplication No. 60/342,072 filed on Dec. 18, 2001 for inventors Reed D.Hanson, Nathaniel B. Wilson, John C. Morris and Timothy F. Ellis andentitled “ALGORITHM TO TRACK RADIALLY-DEPENDENT REPEATABLE RUNOUT.”

FIELD OF THE INVENTION

[0002] The present invention relates generally to servo systems in discdrives. In particular, the present invention relates to compensation forerrors in servo systems.

BACKGROUND OF THE INVENTION

[0003] Disc drives read and write information along concentric tracksformed on discs. To locate a particular track on a disc, disc drivestypically use embedded servo fields on the disc. These embedded fieldsare utilized by a servo sub-system to position a head over a particulartrack. In current disc drives, the servo fields are written onto thedisc in-situ (i.e., after the disc is mounted on the spindle motor of adisc drive) when the disc drive is manufactured and are thereaftersimply read by the disc drive to determine position.

[0004] Ideally, a head following the center of a track moves along aperfectly circular path around the disc. However, various types oferrors prevent heads from following this ideal path. One type of erroris a written-in error that arises during creation of the servo fields.Written-in errors occur because the write head used to produce the servofields does not always follow a perfectly circular path due tounpredictable pressure effects on the write head from the aerodynamicsof its flight over the disc, and from vibrations in the gimbal used tosupport the head. Because of these written-in errors, a head thatperfectly tracks the path followed by the servo write head will notfollow a circular path. Written-in errors are often referred to asrepeatable run-out (RRO) errors or written-in repeatable run-out(WI-RRO) errors because they cause the same errors each time the headpasses along a track. In drives employing in-situ-written discs, the RROor WI-RRO phenomenon is typically not radially-dependent, i.e., there isno definite correlation between the radial position of a track betweenthe disc inner diameter (ID) and the disc outer diameter (OD) on thedisc surface and the WI-RRO associated with the track.

[0005] To meet the demand for greater recording density in disc drives,servo-track writing is undergoing a fundamental change. In the nearfuture, manufactured disc drives will include discs with servo-tracksthat are pre-written onto the discs before the discs are mounted on thespindle motor of the drive. Tests have shown that when such discs withpre-written tracks (pre-written discs) are mounted and clamped on aspindle motor of a disc drive, in addition to WI-RRO errors, RRO errorsalso occur due to centering misalignment of the pre-written servo tracksand the center of rotation of the spindle, and further due to trackdistortion caused by disc clamping forces. This additional RRO inducedin drives including pre-written discs has been found to beradially-dependent, i.e., this additional RRO varies coherently acrossthe surface of the disc from the OD to the inner ID.

[0006] Current servo tracking systems, which are utilized within-situ-written discs described above, are designed for tracking WI-RROand are not suitable for tracking radially-dependent repeatable run-out(RD-RRO). Thus, when such servo systems are employed for head positioncontrol in drives with pre-written discs, the settle time required forthe head before it can properly follow a destination or target track atthe end of a seek operation is relatively large. This large settle time,which is due to the slow adaptation of the servo system to the RD-RRO,negatively impacts the performance of the disc drive.

[0007] Embodiments of the present invention provide solutions to theseand other problems, and offer other advantages over the prior art.

SUMMARY OF THE INVENTION

[0008] The present embodiments relate to disc drive servo systems thatemploy a radially-dependent repeatable run-out tracking scheme to trackradially-dependent repeatable run-out in the servo system, therebyaddressing the above-mentioned problems.

[0009] An apparatus and method for tracking radially-dependentrepeatable run-out in a disc drive having a servo loop for positioning ahead over a rotating disc is provided. The disc includes multipletracks. Radially-dependent repeatable run-out control components for atleast a subset of the multiple tracks are first determined. Datarepresentative of the radially-dependent repeatable run-out controlcomponents for the subset of the multiple tracks is then stored. Thestored data representative of the radially-dependent repeatable run-outcontrol components is utilized to follow different tracks of themultiple tracks.

[0010] Other features and benefits that characterize embodiments of thepresent invention will be apparent upon reading the following detaileddescription and review of the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is an isometric view of a disc drive.

[0012]FIG. 2 is a top view of a section of a disc with pre-written servotracks.

[0013]FIG. 3-1 is a block diagram of a servo loop.

[0014]FIG. 3-2 is an RRO spectrum measured for a drive within-situ-written discs and employing a RRO tracking scheme.

[0015]FIG. 3-3 is an RRO spectrum measured for a drive with pre-writtendiscs and employing a RRO tracking scheme.

[0016]FIG. 4-1 is a block diagram of a servo loop of an embodiment ofthe present invention.

[0017]FIG. 4-2 is an RRO spectrum measured for a drive with pre-writtendiscs and employing an RRO compensation module of the present invention.

[0018] FIGS. 5-1 through 5-4 are plots representing RRO component valuesfor different tracks.

[0019]FIG. 6 is a flow chart representing a method of tracking RD-RRO ina disc drive in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0020] Referring now to FIG. 1, an isometric view of a disc drive 100 inwhich embodiments of the present invention are useful is shown. The samereference numerals are used in various figures to represent the same orsimilar elements. Disc drive 100 includes a housing with a base 102 anda top cover (not shown). Disc drive 100 further includes a disc pack106, which is mounted on a spindle motor (not shown) by a disc clamp108. Disc pack 106 includes a plurality of individual discs, which aremounted for co-rotation about central axis 109. Each disc surface has anassociated disc head slider 110 which is mounted to disc drive 100 forcommunication with the disc surface. In the example shown in FIG. 1,sliders 110 are supported by suspensions 112 which are in turn attachedto track accessing arms 114 of an actuator 116. The actuator shown inFIG. 1 is of the type known as a rotary moving coil actuator andincludes a voice coil motor (VCM), shown generally at 118. Voice coilmotor 118 rotates actuator 116 with its attached heads 110 about a pivotshaft 120 to position heads 110 over a desired data track along anarcuate path 122 between a disc inner diameter 124 and a disc outerdiameter 126. Voice coil motor 118 is driven by servo electronics 130based on signals generated by heads 110 and a host computer (not shown).

[0021] Referring now to FIG. 2, a top view of a section 200 of a disc,with pre-written servo-tracks such as 202, 203 and 204, which is mountedon a disc drive spindle motor having a spin-axis 208 is shown. Disc 200includes a plurality of radially extending servo fields such as servofields 210 and 212 which define a plurality of servo sectors. Disc 200may also be divided into zones, with each zone including multipletracks. In FIG. 2, three zones 214, 216 and 218 are shown. Pre-writtenservo tracks 202, 203 and 204 have an actual track center shown byreference numeral 206. If the track center of the disc coincides withthe center of the spindle motor and if the tracks are perfectlycircular, there will be no repeated position errors or RRO errorsoccurring each time the head passes a particular circumferentiallocation on the disc. However, since the tracks are never perfectlycircular, WI-RRO always occurs in drives. Further, as can be seen inFIG. 2, in a drive with a pre-written disc such as 200, an incongruitybetween the track center 206 and the spindle motor spin-axis 208typically exists. Additionally, in such drives with pre-written discs,servo-track distortion occurs when the disc is clamped onto the spindlemotor. The centering misalignment of the pre-written servo tracks suchas 202, 203 and 204 as well as servo track distortion due disc clampingforces contributes significantly to the RRO phenomenon. The RRO causedby centering misalignment and clamping forces has been found to beradially-dependent and thus varies coherently across the surface of thedisc from the OD to the ID.

[0022] Under the present invention, an RD-RRO tracking scheme isemployed to track RD-RRO in a disc drive. Here, the tracking of RD-RROis carried out by determining RD-RRO control components for the servotracks and storing data representing these RD-RRO control components.This stored data is utilized to follow the servo tracks.

[0023] Referring now to FIG. 3-1, a block diagram of a servo loop 300 isshown. The servo loop 300 includes a servo controller 302 and disc driveactuator mechanics 304. Servo controller 302 is the servo controllercircuitry within internal circuit 130 of FIG. 1. Drive actuatormechanics 304 includes actuator assembly 116, voice coil motor 118,track accessing arm 114, suspension 112 and sliders 110, all of FIG. 1.

[0024] Servo controller 302 generates a control current 306 that drivesthe voice coil motor of drive actuator 304. In response, the driveactuator 304 produces head motion 308. In FIG. 3-1, the RRO error isrepresented as a separate input signal 310 even though the RRO wouldotherwise appear implicitly in head motion 308. The separation of RROfrom head motion 308 provides a better understanding of the presentinvention. In addition, noise in the servo system has been separated andappears as noise 312, which is added to the control signal. The sum ofhead motion 308, which includes noise 312, and RRO 310 results in thehead's servo measurement signal, represented by reference numeral 316.Servo measurement signal 316 is subtracted from a reference signal 318,which is generated by internal circuitry 130 based on a desired locationof the head. Subtracting head measurement 316 from reference signal 318produces a position error signal (PES), represented by reference numeral320, which is input to servo controller 302.

[0025] PES 320 includes an RRO error component and a non-repeatablerun-out (NRRO) error component. As mentioned above, in drives includingin-situ written discs, WI-RRO is caused by imperfectly writtenservo-tracks, and in drives including discs with pre-written servotracks, additional RD-RRO occurs due to misalignment of the track centerof the disc and the spindle-axis, and due to servo track distortioncaused by disc clamping forces. NRRO is caused by spindle ball bearingdefects, rocking modes, disc vibration, etc.

[0026] As can be seen in FIG. 3-1, servo controller 302 includes aWI-RRO tracking module 322 and an NRRO tracking module 324. WI-RROtracking module 322 extracts the RRO component(s) from the PES andoutputs an RRO control signal 323. Similarly, NRRO tracking module 324extracts the NRRO component(s) from the PES and outputs an NRRO controlsignal 325. Control signals 323 and 325 are added to provide controlsignal 306. WI-RRO tracking module 322 is designed for use with drivesthat include discs with in-situ-written servo-tracks and does notfunction efficiently when employed in drives with discs that havepre-written servo tracks. An example feed-forward algorithm is employedin WI-RRO tracking module such as 322, and results obtained fromseek/settle operations by employing such a WI-RRO tracking scheme indrives with in-situ-written discs and drives with pre-written discs aredescribed below in connection with equations 1 through 3 and FIGS. 3-2and 3-3.

[0027] RRO components from rotation of the spindle motor dominate at thefirst few harmonics of the spindle frequency. One feed-forward algorithmthat produces an RRO control signal, i_(f), used to track the f^(th)spindle harmonic is generated as

i _(f) =a _(f)(n)sin(f·θ _(k))+b _(f)(n)cos(f·θ _(k))  Equation 1

[0028] where n is the index of the spindle rotation, and k is the indexfor the servo sector. Coefficients a_(f)(n) and b_(f)(n) are updatedonce per spindle rotation as $\begin{matrix}{{{a_{f}(n)} = {{a_{f}\left( {n - 1} \right)} + {g_{f}{\sum\limits_{k = 0}^{N - 1}{{\sin \left( {f \cdot \theta_{k}} \right)}{{PES}(k)}}}}}}{and}} & {{Equation}\quad 2} \\{{b_{f}(n)} = {{b_{f}\left( {n - 1} \right)} + {g_{f}{\sum\limits_{k = 0}^{N - 1}{{\cos \left( {f \cdot \theta_{k}} \right)}{{PES}(k)}}}}}} & {{Equation}\quad 3}\end{matrix}$

[0029] where N is the number of sectors, g_(f) is the feedforward gain,and PES is the position error signal.

[0030] The algorithm described above, which implements Equations 1, 2and 3, does not perform adequately in drives with large RD-RRO. This isillustrated by the plots shown in FIGS. 3-2 and 3-3. FIG. 3-2 shows RROspectrum 350, with horizontal axis 354 indicating frequency in Hertz andvertical axis 352 indicating magnitude in microinches, collected on anin-situ-written drive, and FIG. 3-3 shows RRO spectrum 360 for a drivewith pre-written discs (with large radially-dependent RRO). Both plotsrepresent spectrums resulting from 200 random seeks. After each seek,the PES signal was collected over eight revolutions and the RRO wassubsequently computed. In each drive, the algorithm described above wasemployed. The first harmonic component, 1f, represented by referencenumeral 356 (FIG. 3-2) and second harmonic component, 2f, represented byreference numeral 358 (FIG. 3-2), which represent a substantial portionof the RD-RRO, are relatively small in the case of the drive within-situ-written discs. However, the 1f and 2f harmonic components 362and 364 (FIG. 3-3) are relatively large in the drive with prewrittendiscs, thereby demonstrating that the algorithm is unsuitable for suchdrives.

[0031] To obtain adequate performance in drives with large RD-RRO, thepresent invention includes an RRO tracking module that is capable ofutilizing stored data representative of radially-dependent RRO toproduce a suitable RRO control signal when the drive switches from trackseek mode to track following mode. Referring now to FIG. 4-1, a blockdiagram of a servo loop of the present invention is shown. In FIG. 4-1,the elements common to FIG. 3-1 are numbered the same. Controller 402,of servo loop 400, is designed to track the relatively large RD-RRO indrives with pre-written discs.

[0032] As can be seen in FIG. 4-1, controller 402 includes RRO trackingmodule 422 and NRRO tracking module 324. RRO tracking module 422includes a first input 432 for receiving PES 320 and a second input 430capable of receiving data representative of RD-RRO components,represented by reference numeral 426. RRO tracking module 422 includesRD-RRO tracking module 424 and WI-RRO tracking module 425. Modules 424and 425 may be integrated or separate. Data 426 is obtained eitherduring factory calibration or start-up calibration of the drive. Anexample calibration procedure for obtaining data 426 is describedfurther below. Data 426 may be stored in the form of a table in memory(for example, non-volatile memory) contained in servo electronics 130(FIG. 1). Depending upon the particular track to be followed, RROtracking module 422 selects a suitable data value from tracking data 426and responsively produces an RRO control signal 423 of appropriatemagnitude and phase for injection into the servo loop. Control signal423 includes a RD-RRO signal component and a WI-RRO signal component. Inone embodiment of the present invention, a suitable RD-RRO data value isselected at the beginning of a seek operation for a destination track sothat the radially-dependent RRO can be tracked as soon as the headarrives at the destination track, thereby substantially reducing thesettle time. In some embodiments, the RD-RRO components are determinedand stored for a subset of the tracks on the disc (less than all of thetracks on the disc). If RD-RRO components are determined and stored fora subset of tracks and if no RD-RRO data is available in stored data 426for a particular track to be followed, then RD-RRO data associated witha track closest to the track to be followed is utilized by trackingmodule 422. Each track of the subset of tracks for which RD-RROcomponents are determined are preferably spaced evenly apart between theID and OD of the disc. In some embodiments of the present invention,data representative of RD-RRO components 426 is obtained and stored foronly one track in each zone. The data for one track within a zone can beutilized for RD-RRO tracking of other tracks within the zone since theRD-RRO varies coherently across the surface of the disc from the OD tothe ID. For example, referring to FIG. 2, RD-RRO data can be obtainedonly for track 202 in zone 214, for track 203 in zone 216 and for track204 in zone 218 and utilized for tracking RD-RRO for all tracks on disc200. NRRO tracking module 324 and the remaining elements of servo loop400 are similar to the elements of servo loop 300. An examplefeed-forward algorithm employed in RRO tracking module 422 of thepresent invention is described below in connection with Equation 4 andFIG. 4-2.

[0033] Large radially-dependent RRO can be tracked by a feed forwardalgorithm described in Equation 4. Here, RRO control signal, i_(f1),used to track the f^(th) spindle harmonic is generated as

i _(f1)=(A _(f)(track_id)+a _(f)(n))sin(f·θ _(k))+(B _(f)(track_id)+b_(f)(n))cos(f·θ _(k))  Equation 4

[0034] For this algorithm, terms a_(f)(n) and b_(f)(n) are updated inthe same manner described in Equations 2 and 3, and A_(f)(track_id) andB_(f)(track_id) represent RD-RRO components or terms. TermsA_(f)(track_id) and B_(f)(track_id) can be implemented as a polynomialcurve fit or a table-lookup scheme.

[0035] The algorithm described by Equation 4 was implemented in a drivebuilt with pre-written discs to track large 1f (first harmonic) and 2f(second harmonic) RD-RRO components. The RRO spectrum 450 generated from200 random seeks on this drive is shown in FIG. 4-2. The drive employedhere is the same type of drive that was used to generate the dataplotted in FIG. 3-3. Thus, the difference in tracking performance can beattributed to the use of the algorithm described by Equation 4.Comparing FIGS. 3-3 and 4-2, it can be seen that a substantialimprovement in tracking the 1f and 2f components (reference numbers 452and 454) resulted from using this algorithm. Also, comparing FIGS. 3-2and 4-2, it can be seen that the level of tracking obtained using thisalgorithm is comparable to what was obtained in drives within-situ-written discs.

[0036] In the above experimental implementation of the algorithmdescribed by Equation 4, the procedure used to calibrate A_(f)(track_id)and B_(f)(track_id) involved the use of the algorithm described byEquations 1 through 3 to determine a_(f)(n) and b_(f)(n) during thecalibration procedure. Steady-state values for a_(f)(n) and b_(f)(n)were read from 30 equally spaced tracks from OD to ID. FIGS. 5-1 through5-4 illustrate plots for a_(f)(n) and b_(f)(n) for the first and secondspindle harmonics, 1f and 2f. In FIGS. 5-1 through 5-4, horizontal axis502 represents track identification number and vertical axis 504represents scaled current. Plot 506 (FIG. 5-1) represents a₁(n)(a_(f)(n) for the first harmonic) and plot 508 (FIG. 5-2) representsb₁(n) (b_(f)(n) for the first harmonic). Similarly, plot 510 (FIG. 5-3)represents a₂(n) (a_(f)(n) for the second harmonic) and plot 512 (FIG.5-4) represents b₂(n) (b_(f)(n) for the second harmonic). A fourthdegree polynomial was fit to each of plots 506, 508, 510, and 512 wasobtained and the resulting polynomials were used to determineA₁(track_id) represented by plot 514 (FIG. 5-1), B₁(track_id)represented by plot 516 (FIG. 5-2), A₂(track_id) represented by plot 518(FIG. 5-3) and B₂(track_id) represented by plot 520 (FIG. 5-4).

[0037] A least-squares polynomial fit method for determiningA_(f)(track_id) and B_(f)(track_id) is descried below in connection withEquations 5 through 11. The description of this method is limited tocomputing the coefficients for A₁(track_id), but the method used tocompute the coefficients for the other polynomials for A_(f)(track_id)and B_(f)(track_id) is identical. Let the desired polynomial forA₁(track_id) be described as

A ₁(x)=c ₀ +c ₁ x+c ₂ x ² +. . . +c _(n) x ^(n)  Equation 5

[0038] where x in the normalized track ID computed as

x=track_id/track_normilization_constant  Equation 6

[0039] During a calibration process, the steady state values for a₁(n)are read at m predetermined track locations, to form m ordered pairs(x_(i),y_(i)) where

y ₁ =a ₁(n)@x ₁  Equation 7

[0040] A least-squares solution for computing the polynomialcoefficients can be computed as

C=XY  Equation 8

[0041] where

C=[c ₀ c ₁ c ₂ c ₃ c ₄]′  Equation 9

[0042] $\begin{matrix}{X = {\begin{bmatrix}k & {\sum\limits_{i = 1}^{k}x_{i}} & {\sum\limits_{i = 1}^{k}x_{i}^{2}} & \cdots & {\sum\limits_{i = 1}^{k}x_{i}^{n}} \\{\sum\limits_{i = 1}^{k}x_{i}} & {\sum\limits_{i = 1}^{k}x_{i}^{2}} & {\sum\limits_{i = 1}^{k}x_{i}^{3}} & \cdots & {\sum\limits_{i = 1}^{k}x_{i}^{n + 1}} \\{\sum\limits_{i = 1}^{k}x_{i}^{2}} & {\sum\limits_{i = 1}^{k}x_{i}^{3}} & {\sum\limits_{i = 1}^{k}x_{i}^{4}} & \cdots & {\sum\limits_{i = 1}^{k}x_{i}^{n + 2}} \\\vdots & \vdots & \vdots & ⋰ & \vdots \\{\sum\limits_{i = 1}^{k}x_{i}^{n}} & {\sum\limits_{i = 1}^{k}x_{i}^{n + 1}} & {\sum\limits_{i = 1}^{k}x_{i}^{n + 2}} & \cdots & {\sum\limits_{i = 1}^{k}x_{i}^{2n}}\end{bmatrix}^{- 1}{\quad {and}}}} & {{Equation}\quad 10} \\{Y = \left\lbrack \begin{matrix}{\sum\limits_{i = 1}^{k}y_{i}} & {\sum\limits_{i = 1}^{k}{x_{i}y_{i}}} & {\sum\limits_{i = 1}^{k}{x_{i}^{2}y_{i}}} & \cdots & \left( \left. {\sum\limits_{i = 1}^{k}{x_{i}^{4}y_{i}}} \right\rbrack \right)^{\prime}\end{matrix} \right.} & {{Equation}\quad 11}\end{matrix}$

[0043] Since the x's contained within the matrix X in Equation 10 aboveare fixed predetermined values, the matrix X (Equation 10) can becomputed offline and stored in memory. Further, matrix X (Equation 10)is common between all calculations for polynomials A_(f)(track_id) andB_(f)(track_id). Additionally, the matrix X (Equation 10) would becommon for all drives within a drive platform. Thus, the least-squarespolynomial fit method is adaptable and involves the storage of arelatively small amount of data. However, since this method involves thestorage of only the polynomial coefficients, A_(f)(track—id) andB_(f)(track—id) have to be computed from the coefficients during seekoperations. In contrast, in a table-lookup method, which involves thestorage of a relatively large amount of data, the values forA_(f)(track—id) and B_(f)(track—id) are stored in a table in memory andare simply read before a seek operation. The values for A_(f)(track—id)and B_(f)(track—id) are steady state values of a_(f)(n) and b_(f)(n)determined for different tracks of the disc.

[0044] Data for the least-squares polynomial fit method or thetable-lookup method can be obtained during a factory calibrationprocedure that is carried out during manufacture of the disc drive, astartup calibration procedure that is carried out during initial startupof the disc drive or a refined calibration procedure that is carried outsubsequent to the initial startup of the disc drive.

[0045]FIG. 6 is a flow chart representing a method of compensating forradially-dependent repeatable run-out in a disc drive having a servoloop for positioning a head over a rotating disc in accordance with anillustrative embodiment of the present invention. The rotating disc hasmultiple servo tracks. At step 602, radially-dependent repeatablerun-out control components for a subset of a plurality of tracks of thedisc are determined. At step 604, data representative of theradially-dependent repeatable run-out control components for the subsetof the plurality of tracks is stored. At step 606, the stored datarepresentative of the radially-dependent repeatable run-out controlcomponents is utilized to follow different tracks of the plurality oftracks. Different techniques, some of which are set forth above, can beemployed to carry out the steps shown in the flow chart of FIG. 6 whilemaintaining substantially the same functionality without department fromthe scope and spirit of the present invention.

[0046] In summary, a method of compensating for radially-dependentrepeatable run-out in a disc drive (such as 100) having a servo loop(such as 400) for positioning a head (such as 110) over a rotating disc(such as 200) is provided. The disc (such as 200) includes a pluralityof tracks (such as 202, 203 and 204). Radially-dependent repeatablerun-out control components for at least a subset of the plurality oftracks (such as 202, 203 and 204) are first determined. Datarepresentative of the radially-dependent repeatable run-out controlcomponents (such as 426) is then stored. The stored data representativeof the radially-dependent repeatable run-out control components (such as426) is retrieved before settling on the target track, and subsequentlyused to follow the selected track (such as 202, 203 and 204).

[0047] It is to be understood that even though numerous characteristicsand advantages of various embodiments of the invention have been setforth in the foregoing description, together with details of thestructure and function of various embodiments of the invention, thisdisclosure is illustrative only, and changes may be made in detail,especially in matters of structure and arrangement of parts within theprinciples of the present invention to the full extent indicated by thebroad general meaning of the terms in which the appended claims areexpressed. For example, the particular elements may vary depending onthe particular application for the servo system while maintainingsubstantially the same functionality without departing from the scopeand spirit of the present invention. In addition, although the preferredembodiment described herein is directed to a servo loop for a disc drivesystem, it will be appreciated by those skilled in the art that theteachings of the present invention can be applied to other systems,without departing from the scope and spirit of the present invention.Further, the radially-dependent RRO tracking scheme may be implementedin hardware or software. The disc drive can be based upon magnetic,optical, or other storage technologies and may or may not employ aflying slider.

What is claimed is:
 1. A runout tracking method for positioning a headover a rotatable disc surface in a disc drive, the method comprisingsteps of: (a) storing several repeatable run-out control componentsdetermined from a plurality of tracks on the disc surface; (b)initiating a seek to a selected target track on the disc surface; (c)before settling on the target track, retrieving the control componentsstored in the storing step (a); and (d) using the control componentsretrieved in the retrieving step (c) to follow the target track.
 2. Themethod of claim 1 wherein the retrieving step (c) is begun beforereaching the target track.
 3. The method of claim 1 wherein the seekincludes a head motion that begins before the retrieving step (c)begins.
 4. The method of claim 1 wherein the retrieving step (c) iscompleted during the seek to the target track.
 5. The method of claim 1wherein the storing step (a) includes a step (a1) of storing datarepresentative of the control components in a nonvolatile memory.
 6. Themethod of claim 1 wherein the storing step (a) includes a step (a1) ofderiving the control components as several polynomial coefficients. 7.The method of claim 1 wherein the control components are determined froma calibration procedure.
 8. The method of claim 7 wherein thecalibration procedure is a factory calibration procedure that is carriedout during manufacture of the disc drive.
 9. The method of claim 7wherein the calibration procedure is carried out during an initial fieldstartup of the disc drive.
 10. The method of claim 7 wherein thecalibration procedure is carried out subsequent to an initial fieldstartup of the disc drive.
 11. The method of claim 1 wherein theplurality of tracks is selected so as to be uniformly distributedbetween an inner diameter and an outer diameter of the disc surface. 12.The method of claim 1 wherein the seek includes a head motion thatbegins before the retrieving step (c) begins.
 13. The method of claim 1wherein the using step (d) is performed by injecting the controlcomponents into a servo loop of the disc drive.
 14. A disc drivecomprising: an actuator configured to move at least one head responsiveto a received servo control signal; a sensor configured to sense servoinformation and to produce a servo signal therefrom, the servo signal iscombined with a reference signal to produce a position error signal; aservo controller which receives the position error signal and generatesthe servo control signal in response to the received position errorsignal, the servo controller comprising: a repeatable run-out trackingmodule adapted to track coherent repeatable run-out in the servo loopby: (a) storing several repeatable run-out control components determinedfrom a plurality of tracks on the disc surface; (b) receiving a commandinitiating a seek to a selected target track on the disc surface; (c)before settling on the target track, retrieving the control componentsstored in the storing step (a); and (d) using the control componentsretrieved in the retrieving step (c) to follow the target track.
 15. Theapparatus of claim 14, further comprising a non-volatile memory suitableto hold data representative of the control components.
 16. A disc drivecomprising: a disc mounted on a spindle motor, the disc having aplurality of tracks; and tracking means, in a servo loop, for trackingcoherent repeatable run-out in the servo loop.
 17. The apparatus ofclaim 16 wherein the tracking means comprises a repeatable run-outtracking module adapted to track coherent repeatable run-out in theservo loop by: (a) determining a coherent repeatable run-out controlcomponents for a subset of the plurality tracks; (b) storing datarepresentative of the coherent repeatable run-out control components forthe subset of the plurality of tracks; and (c) utilizing the stored datarepresentative of the coherent repeatable run-out control components tofollow different tracks of the plurality of tracks.
 18. The apparatus ofclaim 16 wherein the tracking means comprises a repeatable run-outtracking module adapted to track coherent repeatable run-out in theservo loop by: (a) storing several repeatable run-out control componentseach determined from a plurality of tracks on the disc surface; (b)receiving a command initiating a seek to a selected target track on thedisc surface; (c) before settling on the target track, retrieving thecontrol components stored in the storing step (a); and (d) using thecontrol components retrieved in the retrieving step (c) to follow thetarget track.